Upper limits on the escape fraction of ionizing radiation from galaxies at $2\lesssim z < 6$

TL;DR

This study sets upper limits on the escape fraction of ionizing radiation from galaxies at redshifts 2 to 6, combining new observations with literature data, and explores potential correlations with Lyman alpha properties.

Contribution

It provides the most stringent upper limits to date on the escape fraction of ionizing photons from high-redshift galaxies, including a new sample and a comprehensive literature review.

Findings

Upper limit of $f_{esc}^{abs} \,\leq\, 0.06$ for a sample of 9 galaxies.

Combined upper limit of $f_{esc}^{abs} \,\leq\, 0.084$ for non-LyC leakers.

Overall upper limit of $f_{esc}^{abs} \,\leq\, 0.088$ for all examined galaxies.

Abstract

In this work, we investigate upper limits on the global escape fraction of ionizing photons ($f_{\rm esc/global}^{\rm abs}$) from a sample of galaxies probed for Lyman-continuum (LyC) emission characterized as non-LyC and LyC leakers. We present a sample of 9 clean non-contaminated (by low redshift interlopers, CCD problems and internal reflections of the instrument) galaxies which do not show significant ($>$ $3\sigma$) LyC flux between 880\AA\ $<\lambda_{rest}<$ 910\AA. The 9 galaxy stacked spectrum reveals no significant LyC flux with an upper limit of $f_{\rm esc}^{\rm abs} \leq 0.06$. In the next step of our analysis, we join all estimates of $f_{\rm esc}^{\rm abs}$ upper limits derived from different samples of $2\lesssim z < 6$ galaxies from the literature reported in last $\sim$20 years and include the sample presented in this work. We find the $f_{\rm esc}^{\rm abs}$ upper limit $\leq$ 0.084 for the galaxies recognized as non-LyC leakers. After including all known detections from literature $f_{\rm esc/global}^{\rm abs}$ upper limit $\leq$ 0.088 for all galaxies examined for LyC flux. Furthermore, $f_{\rm esc}^{\rm abs}$ upper limits for different groups of galaxies indicate that the strongest LyC emitters could be galaxies classified as Lyman alpha emitters. We also discuss the possible existence of a correlation among the observed flux density ratio $(F_{\nu}^{LyC}/F_{\nu}^{UV})_{\rm obs}$ and Lyman alpha equivalent width EW(Ly$\alpha)$, where we confirm the existence of moderately significant correlation among galaxies classified as non-LyC leakers.